Effect of high-multiple water injection on rock pore structure and oil displacement efficiency

Experimental methods,including mercury pressure,nuclear magnetic resonance(NMR)and core(wateroil)displacement,are used to examine the effects of high-multiple water injection(i.e.water injection with high injected pore volume)on rock properties,pore structure and oil displacement efficiency of an oilfield in the western South China Sea.The results show an increase in the permeability of rocks along with particle migration,an increase in the pore volume and the average pore throat radius,and enhanced heterogeneity after high-multiple water injection.Compared with normal water injection methods,a high-multiple water injection is more effective in improving the oil displacement efficiency.The degree of recovery increases faster in the early stage due to the expansion of the swept area,and the transition from oil-wet to water-wet.The degree of recovery increases less in the late stage due to various factors,including the enhancement of heterogeneity in the rocks.Considering both the economic aspect and the production limit of water flooding,it is recommended to adopt other technologies to further enhance oil recovery after 300 PV water injection.

Microscopic production characteristics of crude oil in nano-pores of shale oil reservoirs during CO_(2)huff and puff

The parameters such as pore size distribution,specific surface area and pore volume of shale rock samples are analyzed by low-temperature nitrogen adsorption experiment,and then the conversion coefficient between relaxation time(T_(2))and pore size is calibrated.Nuclear magnetic resonance experiments of CO_(2)huff and puff in shale samples are carried out to study the effects of gas injection pressure,soaking time and fractures on the oil production characteristics of shale pores from the micro scale.The recovery degrees of small pores(less than or equal to 50 nm)and large pores(greater than 50 nm)are quantitatively evaluated.The experimental results show that the recovery degree of crude oil in large pores increases rapidly with the increase of injection pressure under immiscible conditions,and the effect of injection pressure rise on recovery degree of large pores decreases under miscible conditions;whether miscible or not,the recovery degree of crude oil in small pores basically maintains a linear increase with the increase of injection pressure,and the lower size limit of pores in which oil can be recovered by CO_(2)decreases with the increase of gas injection pressure;with the increase of soaking time,the recovery degree of crude oil in large pores increases slowly gradually,while the recovery degree of crude oil in small pores increases faster and then decelerates,and the best soaking time in the experiments is about 10 h;the existence of fractures can enhance the recovery degrees of crude oil in small pores and large pores noticeably.

Geochemical characteristics of reservoir after water and polymer flooding in Xingshugang oilfield of Daqing

Collecting 44 oil-sand specimens of Pu-I Member in two inspection wells before and after polymer flooding in the thirteenth district of Xingshugang oilfield,with experimental analysis,the author obtained the data about oil viscosity,flow,oil saturation and oil displacement efficiency.The result shows that viscous oil predominates in the main remaining oil in Xingshugang oilfield after water flooding with a certain amount of low viscosity oil,high viscosity oil and heavy oil;after polymer flooding,the viscous oil is main ingredient.Compared with water flooding,the low mobility and poor oil can be spread by polymer flooding,expanding the affected area and improving sweep efficiency and oil recovery.The geochemical affecting factors of water flooding and oil displacement efficiency refer to reservoir flow,permeability and the viscosity of residual oil.In the reservoir with permeability from low to high,the polymer flooding efficiency is better than water flooding.It provides the basis for improving the water and polymer flooding efficiency of the Xingshugang oilfield.

Imbibition mechanisms of high temperature resistant microemulsion system in ultra-low permeability and tight reservoirs

Lower-phase microemulsions with core-shell structure were prepared by microemulsion dilution method.The high temperature resistant systems were screened and the performance evaluation experiments were conducted to clarify the spontaneous imbibition mechanisms in ultra-low permeability and tight oil reservoirs,and to direct the field microfracture huff and puff test of oil well.The microemulsion system(O-ME)with cationic-nonionic surfactant as hydrophilic shell,No.3 white oil as oil phase core has the highest imbibition recovery;its spontaneous imbibition mechanisms include:the ultra-low interfacial tension and wettability reversal significantly reduce oil adhesion work to improve oil displacement efficiency,the nanoscale“core-shell structure”formed can easily enter micro-nano pores and throats to expand the swept volume,in addition,the remarkable effect of dispersing and solubilizing crude oil can improve the mobility of crude oil.Based on the experimental results,a microfracture huff and puff test of O-ME was carried out in Well YBD43-X506 of Shengli Oilfield.After being treated,the well had a significant increase of daily fluid production to 5 tons from 1.4 tons,and an increase of daily oil production to 2.7 tons from 1.0 ton before treatment.

Production calculation of the second and tertiary recovery combination reservoirs under chemical flooding

Based on the analysis of the production composition of reservoirs developed by the second&tertiary recovery combination(STRC),the relationship between the overall output of the STRC project and the production level during the blank water flooding stage is proposed.According to the basic principle of reservoir engineering that the“recovery factor is equal to sweeping coefficient multiplied by oil displacement efficiency”,the formula for calculating the ultimate oil recovery factor of chemical combination flooding reservoir was established.By dividing the reservoir into a series of grids according to differen-tial calculus thinking,the relationship between the ultimate recovery factor of a certain number of grids and the recovery de-gree of the reservoir was established,and then the variation law of oil production rate of the STRC reservoir was obtained.The concept of“oil rate enlargement factor of chemical combination flooding”was defined,and a production calculation method of reservoir developed by STRC was put forward based on practical oilfield development experience.The study shows that the oil production enhancing effect of STRC increases evenly with the in crease of the ratio of STRC displacement efficiency to water displacement efficiency,and increases rapidly with the increase of the ratio of recovery degree at flooding mode conversion to the water displacement efficiency.STRC is more effective in increasing oil production of reservoir with high recovery degree.Through practical tests of the alkali free binary flooding(polymer/surfactant)projects,the relative error of the oil production calculation method of STRC reservoir is about±10%,which meets the requirements of reservoir engineering.

Micro-pore structure and oil displacement mechanism analysis for deep zone and low permeability reservoir in Mobei oilfield

The Mobei reservoir is a low-permeability-sandstone reservoir,due to differences in pore geometry,it can be divided into two independent reservoirs:A1 reservoir and A2 reservoir.For better understanding the water flooding development effects of Mobei reservoir,the mercury intrusion porosimetry,water flooding CT scanning and micro-CT scanning experiments are used in this study.The result shows that the reservoir has the strong heterogeneity which is weaken gradually from A1 reservoir to A2 reservoir.Reservoir pore radius is mainly distributed in the 100e200 microns,the throat radius is mainly distributed in the 1e3 micron.The water flooding core experiment in each reservoir shows a short water-free oil production period and rapid water cut after breakthrough.The A2 reservoir core flooding process is similar to piston displacement,the A1 reservoir core flooding process refers to the phenomenon(The fingering phenomenon in the process of core flooding in the A1 reservoir is obvious).The calculated water drive efficiency of the A2 reservoir is 61.2%,which is higher than 49.1%of the A1 reservoir.According to the CT scanning process,the Mobei oilfield has low micro displacement efficiency and the A1 reservoir has a smaller spread(sweep area)and higher residual oil saturation.

Displacement and development characteristics of fire flooding of vertical wells in old heavy oil areas

As the vertical-well fire flooding technology is industrially applied in the steam-injection old heavy oil areas of Xinjiang and Liaohe oilfields,its enhanced oil recovery potential is gradually clear.According to laboratory experiment,field test and reservoir engineering,the displacement characteristics of verticalwell fire flooding in the steam-injection old heavy oil areas are systematically investigated.Laboratory experiments and core data show that the vertical-well fire flooding has significantly high flooding ef-ficiency,no residual oil are remained in the firing front sweeping zone,and the lateral sweep efficiency and ultimate recovery can be achieved.The vertical-well fire flooding is a strategic replacement technology to enhance the recovery greatly.Development characteristics and advantages of areal and linear fire flooding are well investigated,and research results are applied in the design of industrial test plan of fire flooding in the Hongqian block,Xinjiang.The research shows that the linear well pattern has advantages of easy construction of ground facilities and management,less well of stage management,simple matching technology and easy achievement of purposeful control for fire front;the areal well pattern has advantages in reducing the air/oil ratio during the fire flooding period,increasing the total recovery rate of reservoirs and reducing the geological and reservoir management risks.To select well patterns of fire flooding,some factors such as geology,reservoir,fluid properties,oil price,and reservoir development degree should be mainly considered.In the Hongqian block,an improved linear well pattern with a combination of new wells and old wells is applied,this not only absorb experiences of linear well pattern pilot test,but also use advantages of areal well pattern.

The study on exploitation potential of original low-oil-saturation reservoirs

It is preliminary estimated that the proved geological reserves of original low-oil-saturation reservoirs(OLOSRs)of 54 blocks in China are more than 820 million tons.As same as the high water-cut stage of a reservoir after water flooding,the initial water cut of OLOSRS is higher than 80%.The industrial application of EOR technology could efficiently enhance oil recovery rate from 8.4% to 20.1% of reservoirs after water flooding.The suitable measurements of EOR to the OLOSRs probably have a profound impact on achieving higher oilfield reserves and production.In this paper,the main characteristics and reserves potential of OLOSRs have been studied by investigations and laboratory experiments.The relationship between initial oil saturation and displacement efficiency is also analyzed.The feasibility of EOR technologies for different OLOSRs is discussed.The results show that the OLOSRs could be divided into two categories(primary and secondary)by origin of low oil saturation.The two categories contain abundant reserves in China’s major oilfields,but their occurrence states of crude oil are quite different.If the average oil saturation of the OLOSR is defined at 40%,there are 13.1%-54.2%crude oil that could be recovered by using the existing EOR technology,especially in the primary OLOSRs with low permeability and the secondary OLOSRs with high permeability.